The present invention relates to a method for coating glass containers and, more particularly, to a method for coating glass containers of varying shapes with a layer of plastic.
Glass containers, such as glass bottles, have long been used as receptacles for soft drinks and various other commodities since they are superior in many respects to other types of containers. The consumer can see the marketed commodity directly if the glass is transparent. Glass containers can be formed in a wide variety of shapes and sizes. Additionally, glass containers are easily sanitized. Finally, the recent growth in interest in energy conservation and in resource conservation has increased the attractiveness of glass containers since such containers are easily cleaned and sterilized and thus may be recycled easily. Glass containers are relatively sturdy and will hold up through a substantial number of recycling operations, although, after approximately twenty cycles abrasion marks on the glass will show.
One substantial drawback to the glass container is its tendency to shatter on impact with a hard surface, producing sharp glass shards. The safety risk involved with such breakage is heightened by the fact that many glass containers are pressurized (such as bottles containing carbonated beverages) and the shards which result from shattering the glass container will be scattered over a relatively large area.
It has been determined that coating glass bottles with various plastic materials, can significantly reduce the likelihood of breakage and reduce the scattering of glass shards in the event that breakage does occur. It has also been determined that a suitable coating properly applied to glass bottles will reduce the abrasion marks and reduce breakage during the filling and handling process thereby increasing the number of times a glass container can be recycled. Various techniques and materials have been developed, therefore, for coating glass containers such as glass bottles.
Most commonly used are polyvinyl chloride, polyurethane, polystyrene, ethylene-vinyl acetate copolymers and polyvinyl acetate resins; although, others including urethanes are disclosed in the patent literature. See, for example, U.S. Pat. Nos 3,877,969; 3,889,031; 3,864,152; 3,178,049 and 3,823,032; all of which disclose utilizing polyurethane materials as a component of the coating. These patents also disclose various methods of applying the coatings to the glass bottle. In general it can be said that spraying and dipping are the basic methods used. U.S. Pat. Nos. 3,921,575 and 3,734,765 are examples of each type. A good enumeration of such methods is given in U.S. Pat. Nos. 3,825,141 and 3,825,142, where it is stated:
"It is, of course, appreciated that a suitable means of application of the coating material or sheath . . . to inner glass . . . is a necessity and as examples it is suggested that any of the following may be employed depending upon the manufacturer's desire:
a. By spraying the thermoplastic material as a powder, optionally by an electrostatic spraying method, onto the hot external surface of the inner receptacle; PA1 b. By dipping the inner receptacle, maintained at an appropriate temperature, into a fluidized bed of the plastic material in powder form; PA1 c. By dipping the inner receptacle, if desired while hot, into a molten bath of the plastic material or into a solution or a dispersion of such material, or PA1 d. By any other method of providing a sleeve type coating to an inner glass receptacle known in the art."
All of these methods suffer from several disadvantages in regard to the production of a coated shatterproof bottle that can be washed and processed repeatedly, that will resist wear and deterioration even at the points of stress, and where the coating will for extended periods retain its required ability to prevent shattering. One of the reasons for this is that the above-mentioned methods cannot be easily used to control the thickness of the coating layer. Additionally, if a liquid material is used it must be of a type which can be held in the dipping bath or spray container for extended periods of time without hardening.
One possible solution to the uniform coating and differential wear problem is to apply the coating differentially to the different parts of the bottle. This is difficult to do by conventional spraying or dipping techniques. A system which does utilize a differential coating is disclosed in U.S. Pat. No. 3,912,100 to Graham et al. and U.S. Pat. No. 3,950,199 to Lucas. However, in the arrangement disclosed there, thermoplastic is applied to a band around the neck by spraying (U.S. Pat. No. 3,912,100) or with an applicator head (U.S. Pat. No. 3,950,199) and then a heat-shrink sleeve is applied to the rest of the bottle.
It would be desirable to have a system whereby adjacent zones are coated at the same time with differing amounts, thicknesses or compositions. While flow coating methods for applying different compositions are known (see U.S. Pat. No. 3,802,908 to Emmons), in that instance multiple nozzles are used to apply successive layers of thermoplastic coating material to the rotating cylinders. Applicant knows of no system whereby adjacent zones of a glass container are flow coated with a liquid plastic by use of a series of nozzle means.
Accordingly, it can be seen that a need exists for a method and apparatus for applying a controlled plastic coating across the surface of a glass container, whereby substantially numbers of glass containers may be efficiently coated with a plastic such as polyurethane. Additionally, the apparatus should be designed for easy cleaning at the conclusion of the coating process.